The present invention relates to electric solenoids as used in mechanical linear actuators; more particularly, to such solenoids as may be required to operate without regard to orientation; most particularly to such a solenoid having actuation force maximized by minimization of air gaps in the magnetic pathway within the solenoid.
Electric solenoids are well known in electrical engineering and are widely used as actuating components in electromechanical actuators. A typical electric solenoid consists of a plurality of windings of an electric conductor about north and south polepieces. When current is passed through the windings, a characteristic toroidal magnetic field is produced having field lines at the axis which are parallel to the axis. A ferromagnetic armature is slidably disposed in an axial bore in the polepieces. An axial force is exerted by the magnetic field on the armature which tends to displace the armature axially. The strength of such force can be varied by varying the current flowing through the windings. Thus, by attaching the armature to a shaft, a solenoid may be adapted readily to provide linear mechanical actuation of a device to which it is attached. Solenoids are probably the commonest type of such actuators in use today.
The maximum force which may be exerted on the armature is in part a function of the axial size and stability of the cylindrical air gap between the armature and the polepieces. Ideally, the thickness of the air gap is zero, but conversely, the armature must not touch the either of the polepieces. Further, the armature is not spontaneously centered in the bore, and non-axial magnetic vectors within the bore destabilize centering of the armature, resulting in unpredictable variances in the size and shape of the air gap and in the corresponding response of the armature.
It is known in the art to provide a lubricious, non-magnetic, cylindrical sleeve in the air gap to keep the armature centered in both of the polepieces and to function as a journal bearing to facilitate low-friction motion of the armature. Such a sleeve can reduce the centering problem but in itself still contributes to the thickness of the non-magnetic gap between the armature and the polepieces, thus limiting the maximum actuating force of the solenoid. Such a sleeve also has frictional contact, however small, with the armature over the full length thereof, through both polepieces.
Further, because of necessary tolerances between the sleeve and the armature and between the sleeve and the polepieces, the armature may still be unacceptably decentered by gravity if the actuator is used in orientations wherein the actuator axis is inclined more than about 30xc2x0 from vertical. Thus, prior art solenoid actuators can impose serious engineering design restrictions in their use.
Solenoids are inherently inefficient due to their relatively high radial/axial force ratio. Radial forces on the armature exist because the magnetic field within the windings is fully parallel to the axis of the solenoid only at infinite distances from the axial ends of the windings. At all other locations, because of the magnetic fringing field a significant radial component exists which tends to decenter the armature unpredictably and frictionally against the guiding sleeve. Even in solenoids having the best available lubricious coatings of the guiding sleeve, the ratio of radial-to-axial forces can be as high as 10:1. Because only the axial component of force can be utilized to move the armature axially, the radial forces constitute parasitical friction which must be overcome by the device to perform properly.
What is needed is an improved, efficient solenoid which may be used in any orientation without loss in effectiveness, wherein the thickness of the gap between the armature and the polepieces is minimized and controlled to be substantially cylindrical and wherein the reluctance of the magnetic circuit is minimized.
The present invention is directed to an improved solenoid for providing linear actuation. The outer polepiece of the solenoid is provided with an axial, self-lubricated, non-magnetic journal bearing for supporting an actuating shaft extending coaxially from the solenoid armature. Preferably, the radial tolerance between the diameters of the bearing inner bore and the shaft is as small as in practically possible without inducing significant drag of the shaft in the bearing. This feature permits elimination of that prior art portion of the guiding sleeve extending into the outer polepiece, thereby reducing frictional losses with the sleeve, and reduction in thickness of the air gap between the armature and the outer polepiece. Further, small prior art air gaps between the pole pieces and the housing are eliminated to reduce reluctance of the magnetic circuit. A significant increase in actuating force is realized in comparison with a prior art solenoid actuator.